With the spread of the Internet, an amount of data that is communicated is increased. In order to deal with this, there is a need to increase a capacity of a transmission path. As one technology for realizing a greater increase in capacity, there is a multilevel modulation (quadrature amplitude modulation: QAM) scheme. An optical signal on which modulation in compliance with the QAM scheme is performed in a transmitter is demodulated in a digital-coherent type light receiver.
In optical communication in compliance with the QAM scheme, there is a major problem of a nonlinear effect to which the optical signal is subjected upon propagating along the transmission path. When the optical signal is subjected to the nonlinear effect while in the transmission path, a phase of the optical signal is rotated. Because complicated phase information is handled in the QAM scheme, if the optical signal is subjected to phase rotation due to the nonlinear effect, correct phase information cannot be demodulated at the time of receiving.
In contrast, a nonlinear compensation scheme called backpropagation is disclosed in Non-Patent Document 1. This compensation scheme is a scheme in which dispersion compensation is performed little by little and nonlinear compensation is performed immediately after each dispersion compensation operation is performed, and thus waveform distortion is compensated for while a propagation waveform is traced backward from the receiving side to the transmitting side.
However, in the backpropagation, when a dispersion compensation function and a nonlinear compensation function are combined as one nonlinear compensation stage, there is a need to increase the number of stages for the nonlinear compensation. The dispersion compensation function is realized by a linear distortion compensation circuit, and the nonlinear compensation function is realized by a nonlinear distortion compensation circuit. Because the linear distortion compensation circuit performs the dispersion compensation in a frequency domain, the linear distortion circuit includes an FFT/IFFT circuit. Because the FFT/IFFT circuit is large in circuit scale, when a mounting area for an LSI and power consumption are considered, only several FFT/IFFT circuits can be mounted in one signal processing device.
In contrast, a compensation scheme called filtered backpropagation is also disclosed in Non-Patent Document 1. In the filtered backpropagation, an amount of a time average of amounts of phase rotation, which is calculated from a signal strength, is used for the nonlinear compensation, and thus the number of stages for the nonlinear compensation is reduced. Furthermore, in Non-Patent Document 1, a low pass filter is used for the time average of the amounts of phase rotation.
Additionally, a technique for setting a coefficient of the low pass filter described above is disclosed in Non-Patent Document 2. In Non-Patent Document 2, demodulation is performed without the nonlinear compensation being performed on the received optical signal. A difference between a symbol position that is demodulated and an ideal symbol position for the signal is monitored, and thus the coefficient of the low pass filter is determined.